Test system



y 9 6- D. E. GARR ET AL 2,399,918

TEST SYSTEM Filed NOV. 13, 1943 enema y 1, 194a 2,890,928 'ras'r srs'rmr Donald E. Gan. Weaver, Sootia,

Electric Company, a corporation of New York- Applicatlon November 13, 1943, Serial No. 510,235 16 Claims. (01. 73-162) Our invention relates to test systems for testing mechanical drives such as geardifferentials and transmissions.

An object of ourinvention is to provide an improved test system for simultaneously testing a plurality of mechanical drives. I

Another object of our invention is to provide an improved test system for testing a pair ,of mechanical drives such as differential gears and for pumping back the power developed by one mechanical drive into the machine which drives the other mechanical drive, thereby increasing the efilciency of the system.

' Further objects and advantages of our invention will become apparent and our invention will be better understood from the following description referring to the accompanying drawing, and the features of novelty which characterize our invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

The drawing is a schematic illustration of an embodiment of 'our improved test system.

Referring to the drawing, we have shown a pair of mechanical drives, each including a dinerentiai gear drive I which is to be tested. The output shafts ll of these differential drives on each side of each unit are mechanically connected together by any suitable driving arrangement, such as a belt or chain drive l2 for transmitting power therebetween. The input or transmission shafts l3 and I4 of these two gear drives are mechanically connected respectively to the armatures of dynamoelectric machines I! and I8. These dynamoelectric machines are provided with commutat'ing field excitingwindings l1 and II and are connectedin parallel with each other through resistors I9 and 20 and across a common source of electric power supply through contactors 2| and 22. This common source of electric power supply is a main generator having an armature 23 adapted to be driven at a substantially constant speed by a suitable prime resistors l9 and 20, respectively. The dynamoelectric machines 15 and it are provided with control field exciting windings 29 and 30 which are energized by amplidyne exciters 3| and 32.

If the fields 29 and 30 are maintained exactly equal, each dynamoelectric machine will drive its ton and differential at some predetermined speed, depending upon the voltage of the generator 23. As long as the excitations provided by the field exciting windings 23 and 30 are substantially equal and constant, both machines will tend' to run at the same speed and no power will be transmitted through the gear drives. If the excitation of one of the control field exciting windings is weakened such that the other field exciting winding provides a higher excitation to its respective dynamoelectric machine, the machine with the higher excitation will tend to run slower than the machine with the weaker excitation, and mechanical power will be transmitted through the gears from the machine with the weakened field to the machine with the higher excitation, thus transmitting mechanical power through the gear drives. With such an arrangement, the machine with the higher excitation will become a generator, and electric power will be pumped back and transmitted from it to the driving dynamoelectric machine with the weaker field, and the losses of the system will be provided by the main generator 23. This'provides for-a very efiicient utilization of this test system.

In order to assure rapid response and to provide the desired control features to the machines, various field exciting windings are provided to the amplidyne exciters 3| and 32 and the main generator 2-3 is provided with a main control field exciting winding 33 which is energized from an amplidyne exciter 34. All of the amplidyne exciters are of the type machine described in Patent 2,227,992, Aiexanderson and Edwards, January 7, 1941, assigned to the assignee of this applicatiomand all of these exciters are adapted' to be driven at substantially constant speeds by suitable motors 35, 36, and 31. The amplidyne exciters 3|, 32, and 34 are provided with the usual type compensating field exciting windings 38, 39, and 40, respectively, which are connected in series with the armature to compensate or neutralize the load current component of armature reaction. These amplidyne exciters also are I provided withconventional stabilizing or antihunt field exciting windings 4|, 42, and 43, connected through condensers 44, 45, and 48 across the exciters 3|, 32, and 34, respectively. These stabilizing field exciting windings are energized only under transient conditions and provide a component of excitation in a direction which tends to dampen out the transient condition in th exciter. It is desirable to limit the speed of operation of the dynamoelectric machines l5 and 'IE to a predetermined safe maximum value.

This is obtained by arranging a speed limiting field exciting winding 41 on the main exciter 34 which is adapted to be deenergized for all values below the predetermined maximum speed value and to provide a differential component of excitation to the exciter which will tend to lower the voltage of this exciter and thereby lower the excitation provided by the field exciting winding 33 to the main generator 23 when the speed of either machine I5 or it exceeds a predetermined value. When the field exciting winding 41 causes a decrease in the excitation of the main generator 23, the ability of this generator to supply the losses of the driving system including the gear drives and the dynamoelectric machines is reduced such that the machines slow down as the energy is absorbed by the uncompensated losses and the speed of the test system is maintained within safe operating values. This excite: field exciting winding 47 is energized by being con-' nected across a voltage marked on terminals 48 and 49 of full wave rectifiers and marked on terminal I50 a resistor Diametric terminals 52 and 53 of the full wave rectifiers are connected respectively to speed responsive voltages generated by tachometer generators U4 and 55 which are mechanically coupled to the dynamoelectric machines I5 and I8 and are adapted to be driven in a predetermined speed relationship to the speed 01' these two machines, respectively. The two full-wave rectifiers are connected in parallel and in opposition to the separate source 01' power supply across the field exciting winding 41 by having their terminals 58 connected together to a terminal of the field exciting winding 41, such that when the speed responsive voltage of either or the tachometer generators exceeds the voltage of the separate source of supply, a current will flow through the field exciting winding 41 to provide the desired diflerential component of excitation to the main exciter 34. Reverse energization of the field exciting winding 41 is prevented at speeds below the predetermined maximum speed by a unidirectional valve or rectifier 51 which is connected in series with the winding and permits energizing current to flow only when the voltage or the tachometer generators exceeds the voltage of the external source.

It is also desirable to vary the amount of loading 0 the mechanical drives and to reverse the direction of operation 0! the gears to testthe gear mesh for both directions of rotation, and under some conditions it may also be desirable to operate only one or the dynamoelectric machines IS or 18 without operating the other machine. In order to provide this type operation, a manually operable selective control 58 is arranged to control the energization of control field exciting windings of the amplidyne exciters 3|, l2, and 34. This includes a plurality of contactors 59, 60, SI, 62, 63, Stand 85 which are adapted to close circuits for simultaneously reversing the direction of excitation provided by the control field exciting windings or selectively to energize only one or the other of the control field exciting windings of the amplidyne exciters 3| or 32, respectively. 'The energization of the control field exciting windings is provided by an external source of electric power supply which is indicated by and signs in the drawing and excitation is removed from all of the control field exciting windings in the position of the manual selective control 58 shown in the drawing. If only the exciter for the dynamoelectric machine 15 is to be energized, the controller is turned to the first position on the right indicated at A in *the drawing, in which position the contactor 59 is closed on contact 85 and contactor 60 is closed on contact 61, which places the external voltage across a potentiometer connected in series with the exciter 3i and through resistors Ed and 5| in series with a field exciting winding :0 adapted to control the voltage oi the amplidync exciter 3|. The energlzation of the field exciting winding Ill also is manually controllable by a rheostat l i connected in series with this winding. In this position oi! the selective control, the contactors iii, 82, 53, and 6) do not connect with any contact, and, therefore, the control field exciting windings of the amplidyne exciter 32 are deenergized. With such an arrangement, the main circuit switch 22 of the dynamoelectric machine IB also should be opened. Energization of the dynamoelectric machine !5 is provided by the main generator 23. and the main generator field exciting winding 33 is energized by the amplidyne exciter 34, which is excited by its various field exciting windings, includin a control field exciting winding 12. This field exciting winding is energized by the difference of the voltage across a resistor 13 in series with the amplidync exciter armature 34 and the voltage from an external source through the adjustable resistor 89, as the contactor 85 of the selective control is closed on contact 14. With this arrangement, the external voltage is compared to the potentiometer drop, that is, these two voltages are connected in opposition to each other and across the field exciting winding I2. The control excitation provided by this field exciting winding 12 can be regulatedby varying the resistance of the rheostat 88 and also by varying the voltage compared to the external voltage by changing the setting of the potentiometer 13. In this manner, the voltage or the main generator 23 is controlled by the control of the excitation provided by its field exciting winding 33 through control or its energizing current.

Operation of only the dynamoelectrlc machine I. can be obtained by turning the selective control I! to its first position towards the left in the drawing, indicated by B, in which position the main exciter field exciting winding I2 is energized as before by the closing oi the contactor 65 on the contact II which is electrically connected in the same circuit as the contact 14. In thi position, the contacts II, 0, II and 62 are deenergized and do not connect with any electrical contacts. However, the contactors SI and 84 are closed on contacts I8 and 11, respectively, and energize a control field exciting winding II on the exciter 32 in the same manner as the field excltingwinding III or the exciter II, as described above. In this arrangement, the field exciting winding 18 is connected across a variable potentiometer 19 connected in series with the armature of the exciter 82 and across the external source of electric power supply through the rheostat 88 and the resistance 5! and may include rheostat 88. The energization of this field exciting winding 18 also is manually controllable to a small extent by a rheostat connected in series with this winding to provide better regulation to this field winding.

Operation of both of the dynamoelectric machines l5 and I 6 can be obtained from either of the first two positions A or B o! the selective controller 58 by moving to either of the extreme positions on the right or left, indicated by AB and BA, respectively. In the position AB, the contactor 59 is closed on a contact 34 and the contector 60 is closed on a contact 32 which energizes the field exciting winding M in the same manner as when the selective controller 58 i in the position A. The contactor 63 is closed on a contact 83 which energizes the main exciter control field exciting winding 12in the same manner as when the selective controller is in the position A. The contactors 33 and 34 are closed on contacts 34 and 35 which reverses the energlzation of the field exciting winding 13 from the energization provided when the selective controller 58 is in its position B and the energization of this field exciting winding 18 is also controllable by the resistors 38 andin this position, such that the voltage of the exciter 32 is reversed from the voltage of this exciter when a selective controller is in the position B. In this position of the selective controller 33, the contactor II and 32 are closed on contacts 33 and 33 which connect the voltage of the main generator 23 across con. trol field exciting windings 30 and 3| of the exciters 3| and 32, respectively, through manually controllable rheostats 32 and 33 through the contacts of the contactors 2| and 22 in opposition to an external voltage through a full-wave rectiher 94. This is obtained by connecting the terminals of the rheostats 32 and 33 across diametric terminals 38 and 33 of the full-wave rectifier 34 and by connecting the external voltage across the other diametric terminals 31 and 33 through a variable potential transformer 39, the primary of which is connected across a source of alternating current at "it. With this arrangement, the voltage 01' either exciter 3! or 32 can be varied at will by varying the position of the rheostats s2 and 93 to provide more or less excitation to-one or the other of the exciters 3| or 32. Thus, either of the dynamoelectric machines I! or It can be made to act as a motor or a generator, depending upon the amount of excitation provided thereto as controlled by the excitation of the field exciting windings 23 and 30 as a function of'the control of the rheostats 32 and I3; The connections of the field exciting windings 30 and 3| also provide a safe maximum current limit to the dynamoelectric machines I! and I3, as the voltage across the diametric terminals 3! and 33 of the rectifier 94 is proportional to the drop across the resistors l3 and 20, which is proportional to the load current oi the machines II and I3. By varying the amount of the resistances II and 20, the maximum load current of the machines I8 and I3 can be varied as if it exceeds a predetermined amount. The excitation of the field exciting windings 90 and 3| will oppose the excitation of the main control field exciting windings Ill and I3, respectively, thereby decreasing the net excitation and, therefore, the voltage of the exciters 3| and 32. This would reduce the excitation provided by the field exciting windings 29 and 30 and produce a lower voltage and load current for the dynamoelectric machines I! and I3, respectively, thereby limiting the curv rent through these machines to the desired maximum value.

The dynamoelectric machines I! and I3 may be Letters Patent of the United States is:

operated in the reverse direction by turning the and energize the control field exciting winding 10 of the exciter 3| in the reverse direction from that explained above when the selective controller 58 is in its position A. In addition, the contactors 3i and 32 are closed on contacts I06 and It! and energize the control field exciting windn s 30 and 3| of the exciters 3i and 32 in the opposite direction from that explained above for the position of the selective controller 58 in its position AB, thereby reversing the excitation pro vided to the dynamoelectric machine field exciting windings 23 and 30 by the exciters 3i and 32, respectively.

It has been found desirable to limit the current supplied by the main generator 23 to a predetermined maximum safe operating value, and this is obtained by arranging a field exciting winding I38 to provide a diflerential component of excitation to the main exciter 34, above a predetermined value of current supplied by the main generator 23. This field exciting winding I08 is connected across the resistor 26 and across an external voltage through a bridge formed of two resistor arms Hi3 and H0 connected across diametric terminals ill of a full-wave rectifier H2. The other diametric terminals I I3 of the fullwave rectifier I I2 are connected across the secondary I ll of a transformer, whose primary winding H5 is connected to the alternating current source of electric power supply Hill. The tenminals III of the full-wave rectifier are con nected across the resistors I09 and H0 and to the resistance 23 through one-way rectifiers lid and i H which prevent a short circuit of the fullwave rectifier H2 and permit the flow of current through the winding Ill only in one direction, so as to permit the energization of this current limiting field exciting winding only when the voltage drop across the resistor 23 exceeds the drop provided across the-resistor arm II.

In this manner, the dynamoelectric machines I3 and It may be operated independently of each other or both may be operated together in either direction of rotation, so as to reverse the rotational direction of the transfer of mechanical power through the mechanical drives, and either machine may be operated as a motor or as a generator when both machines are in operation. This provides a very versatile control and a very efilcient 'utilization of the power of the system.

While we have illustrated and described a particular embodiment of our invention, modifications thereof will occur to those skilled in the art. We desire it to be understood, therefore, that our invention is not to be limited to the particular arrangement disclosed, and we intend in the appended claims to cover all modifications which do not depart from the spirit and scope of our invention.

What we claim as new and desire to secure by 1. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives, means for electrically connecting both of said machines across a source of electric power supply, a field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric 7 machines for driving said higher excitation mameans for simultaneously reversing the direction of rotation of the mechanical drives by simultaneously reversing the excitation of said machines.

2. In a test system for simultaneously testing a pair of gear drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechanically connected to one oi said gear drives, means for electrically connecting both of said machines across a source of electric power supply, a field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for driving said higher excitation machine by said other machine, said electrical connecting means of said machines to said electrical power supply providing an electrical pump-back or power from said driven machine to said other machine and for supplying the losses of the system from said electrical power supply, means for simultaneously reversing the direction of rotation of said gear drives by simultaneously reversing the excitation of said machines and also for selectively energizing said field exciting winding of only one of said dynamoelectrlc machines, and speed responsive means for limiting the speed of said machines to predetermined values by limiting the supply of power from said electrical power supply.

3. In a test system for testing apair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives, means for electrically connecting both of said machines across a source of electric power supply, a field exciting winding ior each 01' said' dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through the mechanical drives, said electrical connecting means of said machines to said electrical power supply providing for an electrical pump-back power from said driven machine to said other machine and said electrical power supply supplying the losses of the system, and means for simultaneously reversing the direction of rotation of said mechanical drives by simultaneously reversing the excitation oi. said machines and also Ior selectively energizing either one or both oi said field exciting windings of said machines.

4. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetwcen, a pair of dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives, means for electrically connecting both of said machines across a source oi electric power supply, a field exciting Winding for each of said dynamo eiectric machines, means for controlling the excitation. provided by said field exciting windings for operating one of said dynamoelectric machines assessors at a higher excitation than the other of said dynamoelectrio machines ior mechanically driving said higher excitation machine by said other machine through the mechanical drives, said electrical connecting means of said machines to said source of electrical power supply providing for an electrical pump-back of power irom said driven machine to said other machine and said electrical power supply supplying the losses oi the system. means for measuring the electrical power of each of said machines, and means for simultaneously reversing the direction of rotation oi said mechanical drives by simultaneously reversing the excitation of said machines and also for selectively exciting either one or both 0! said machines.

5. In a test system for testing a pair oi mechanical drives connected together ior transmitting power therebetween, a. pair of dyna moelectric machines each adapted to be mechanically connected to one of said mechanical drives.

means for electrically connecting both 01 said machines to a source of electric power supply. 'a field exciting winding for each oi said dynamoelectric machines, means for controlling the excitation provided by said fleld exciting windings for operating one '0! said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for driving said.

higher excitation machine by said other machine through said mechanical drives, said electrical connecting means 01' said machines to said electrical power supply providing for an electrical pump-back of power from said driven machine to said other machine and for supplying the losses of the system from said electrical power supply, selective means for simultaneously reversing the direction of rotation of said mechanical drives by simultaneously reversing the excitation of said machines, and means including 9, held exciting winding for said machines for controlling the loading on said machines.

6. In a test system for testing a pair of mechanical drives connected together for transmitting ppwer therebetween, a pair 01' dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives, a generator. means for electrically connecting both of said machines across said generator, a field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a. higher excitation than the other of said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through the mechanical drives, said electrical connecting means of said machines across said generator providing for an electrical pump-back 01' power from said driven machine to said other machine and said generator supplying the losses of the system, means for simultaneously reversing the excitation of said machines, means including an exclter for exciting said generator, and means including a field exciting winding for providing a diiferential component of excitation to said exciter above a predetermined current supplied by said generator for limiting the generator current to a predetermined maximum value.

7. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives,

a main generator, means for electrically connecting both'of said machines across said main generator, a field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for me-- chanically driving said higher excitation machine by said other machine through said mechanical drives, said electrical connecting means of said machines and said main generator providing for an electrical pump-back of power from said driven machine to said other machine and for supplying the losses of the system from said main generator, means for simultaneously reversing the excitation of said machines for reversing the direction of rotation of said mechanical drives, means including an exciter for exciting said generator, and means including a field exciting winding on said exciter for limiting the maximum current supplied by said main generator.

8. In a test system forsimultaneously testing a pair of mechanical drives connected together for transmitting power therebetween, a pair oi dynamceiectricmachines each adapted to be mechanically connected to one of said mechanical drives, means for electrically connecting both oi said machines across a source of electric power supply, a field exciting winding for each oi said ometer generators abovea predetermined medynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one 01' said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for driving said higher excitation machine by said other machine, said electrical connecting means of said machines I to said electrical power supply providing for an electrical pump-back of power from said driven machine to said other machine and for supplying the losses of the system from said electrical power supply, speed responsive means for limiting the speed of said machines to predetermined values by limiting the supply of power from said electrical power supply, and means ior'simultaneously reversing the direction of rotation oi? said machines to reverse the rotational direction of the transfer of mechanical power through said mechanical drives.

9'. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of vdynamoelectrio machines each adapted to be mechanicallyconnected to oneot said mechanical drives, a main generator,'-means for electrically connecting both of said machines across said generator, at field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other 01' said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through the mechanical drives, said electrical connecting means of said machines to said generator providing for an electrical pump-back of power from said driven machine to said other machine and said generator supplying the losses of the system, means including an exciter forexciting said generator, and means including a tachometer generator on each or said machines and a field exciting winding for providing a component of excitation to said exciter v trical connecting means chine speed for reducing the voltage of said main generator for reducing the speed of said mafchines.

10. Ina-test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechanically A connected to one of said mechanical drives, means for electrically connecting both of said machines across a source of electrical power supply, a field exciting winding for each of said dynamoelectrlc machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for driving said higher excitation machine by said other machine, said electrical connecting means or said machines to said electrical'power supply providing for an electrical pump-back of power from said driven machine to said other machine and for supplying the losses of the system from said' electrical power supply, means for measuring the electrical power oi each oi said machines, speed responsive means for limiting the speed of said machines to predetermined values by limiting the supply of power from said electrical power supply, and means for simultaneously reversing the direction of rotation of said machines to reverse the rotational direction of the transfer of mechanical power through said mechanical drives.

, 11. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair'of dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives, means 'for electrically connecting both of said machines to a source of electric power supply. a field exciting winding for eachoi' said dynamoelectric machines, means for' controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through the mechanical drives, said elecof said machines to said source of electrical power supply providing for an electrical pump-back of power from said driven machine to said other machine and said electrical power supply supplying the losses of the system, means for measuring theelectrical power ofeach of said machines, means for simultaneously reversing the direction of rotation of said mechanical drives, speed responsive means for limiting the speed of said machines to predetermined values by limiting the supply of power from said electrical power supply," means for selectively energizing said field exciting winding of only one of said dynamoelectrlc machines, and means including a field exciting winding for said 'machines for controlling the loading on said machines.

12. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelecating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through said mechanical drives, said electrical connecting means of said machines to said electrical power supply providing for an electrical pump-back of power from said driven machine to said other machine and for supplying the losses of the system from said electrical power supply, means for simultaneously reversing the direction of rotation oi said mechanical drives, means including a field exciting winding for said machines for controlling the loading on said machines, and speed responsive means for limiting the speed of said machines to predetermined values by limiting the supply of power from said electrical power supply.

13. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechan- V ically connected to one of said mechanical drives,

means including a generator for providing a source of electrical power supply, means for electrically connecting both of said machines across said generator, a field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through the mechanical drives, said electrical connecting means ing windingfor providing a differential component of excitation to said exciter above a predetermined current supplied by said generator for limiting the generator current to a predetermined maximum value, and speed responsive means for limiting the speed of said machines to predetermined values by limiting the supply of power from said electrical power supply.

14. Ina test system for testing a pair oi mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives, means including a generator for providing a source of electrical power supply, means for electrically connecting both of said machines across said generator, a field exciting winding for each of said dynamoelectric machines, means for control ling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through the mechanical drives, said electrical connecting means of said machines to said generator providing for an electrical pump-back oi! power from said driven machine to said other machine and said electrical power supply supplying the losses of the system, means including an exciter for exciting said genrent supplied by said generator for limiting the generator current to a predetermined maximum value.

15. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectrie machines each adapted to be mechanically connected to one of said mechanical drives, means for electrically connecting both of said machines to a source of electric power supply, a field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other of said dynamoelectric machines for driving said higher excitation machine by said other machine through said mechanical drives, said electrical connecting means of said machines to said electrical power supply providing for an electrical pump-back of power from said driven machine to said other machine and for supplying the losses of the system from said electrical power supply, means for simultaneously reversing the direction of rotation of said mechanical drives, means including a field exciting winding for said machines for controlling the loading on said machines, means for measuring the electric power of each of said machines, and speed responsive means for limiting the speed of said machines to predetermined values by limiting the supply of power from said electrical power supply.

16. In a test system for testing a pair of mechanical drives connected together for transmitting power therebetween, a pair of dynamoelectric machines each adapted to be mechanically connected to one of said mechanical drives, means including a main generator for providing a source of electrical power supply, means for electrically connecting both of said machines across said main generator, a field exciting winding for each of said dynamoelectric machines, means for controlling the excitation provided by said field exciting windings for operating one of said dynamoelectric machines at a higher excitation than the other .of said dynamoelectric machines for mechanically driving said higher excitation machine by said other machine through the mechanical drives, said electrical connecting means of said machines to said main generator providing Ior an electrical pump-back 0! power from said driven machine to said other machine and said electrical power supply supplying the losses of the system, means including an exciter for exciting said generator, means including a field exciting winding for providing a differential com ponent of excitation to said exciter above a predetermined current supplied by-said generator for limiting the generator current to a'predetermined maximum value, and means including a tachometer generator on each of said machines and a field exciting winding for providing a component of excitation to said exciter above a predetermined machine speed for reducing the 

